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The question is what physical process or rule selects that single reading.","posed_since":"","precise":"For a system $S$ in state sum_k c_k |k> coupled unitarily to apparatus $A$ and environment $E$, unitary evolution gives sum_k c_k |k>|A_k>|E_k>; decoherence makes the reduced density matrix of $S+A$ diagonal but selects no single $k$. Required: a modified dynamics, an added ontology (hidden variables, branches) or a principled account that yields single outcomes with frequencies $\\mid c_k\\mid^2$, together with either predictions that differ from standard quantum mechanics or a proof of empirical equivalence. This is the root problem of the topic; its workable sharper forms are listed under related.","problem_ref":null,"references":"","settled_by":"An experiment detecting a deviation from unitary evolution predicted by one class of proposals, or a proof that a specified account reproduces all quantum predictions using only its stated assumptions.","status_note":"","title":"Why does a measurement produce one definite 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